Planets
that are close to the sun are
made mostly of rocks and
minerals, because minerals
can solidify
at higher temperatures than
gases. It is hot close to the sun, but rocks and
dust can form there, while
the more volatile elements,
the gases, do not condense
until they are beyond the
zone of habitability, the
live zone.

Elements that
we know as gases, such as oxygen and nitrogen, do not become
solid until they are very cold. The outer planets are made of
very cold gases, frozen gases,
and water ice. It is
believed that they may have
rocky cores.
Their moons are also cold, and
seem to be made up of ice
mixed with rocks and
minerals.

The planets
and moons of the inner solar
system, Mercury, Venus,
Earth, and Mars, have solid surfaces. They
are very interesting to
geologists because they are
made of the same elements as
the earth, but have
different histories and
characteristics.

As
a planet forms, gravity draws the the rocks and dust from which
a planet is made together. When enough matter has been gathered,
heat is released, and the planet may become a molten ball of
melted material. While the planet is molten, the heaviest elements
sink toward the center (although some proportion of the heavy
material stays on the surface), and the lighter elements float
to the top.

Eventually the
outside of the planet cools and begins to solidify and form a
crust. As the crust solidifies it contracts, forming an uneven
surface, with mountains, cliffs, rifts, and flat areas. Cracks
appear in the shell of the planet. Hot material from the inside
of the planet may escape, forming volcanoes, and meteors from
space may crash on the planetary surface, creating craters. Steam
may escape from the volcanic vents, and the icy parts of meteors
may melt and add water. While the planet is hot, water exists
as steam and rises in the atmosphere until it can form clouds,
and, later, rain. Some of the water escapes into space, but on
our planet, and possibly others, enough remains to support living
beings.

Meanwhile, the
interior of the planet cools more slowly. Pressure causes the
iron core of the planet to solidify despite its high temperature.
The mantle, under the crust, remains more plastic, allowing the
plates of solid rock on the surface to float and slowly move
about. The places where these tectonic plates meet become sites
of volcanism and mountain building.

Other processes
also affect the appearance of landscapes. Erosion by wind and
water wears the rocks away: sand and pebbles are carried in rivers
and floods. Low lying plains may become bogs and wetlands. Ice
freezes in rocky cracks, splitting the rocks apart. Planets are
constantly undergoing change.

Mapping
your planet will set the stage for the development of your world.
How much of the world will be covered by water? Earth is about
70 per cent covered by water, which ensures plenty of evaporation
for clouds and rainfall. Will your oceans be shallow or deep?
Deep oceans permit better circulation of the water, but shallow
seas can be warm and life nurturing. If your world is a drier
world, will life be restricted to just a few areas? How warm
will your world be? Will there be ice at the poles?

Something else
to consider is the influence of gravity. If the mass of your
world is greater than that of the earth, it will have greater
gravitational forces. Gravity greater than our earth's will require
more force to push mountains up. Mountains will probably not
be so high as they are on earth. Rain will fall faster, and hit
the ground harder. Will this speed up erosion? On the other hand,
a planet with less mass than earth will have less gravity, and
mountains may be higher and steeper. Have fun thinking about
this!

Photograph from a Corel CD-ROM
: for viewing only, not for downloading. More Information.